SOLUTION:

First Step: Invert both the hourglasses.

Second step: When the 11 min hourglass is finished, invert it.

Third Step: When the 13 min hourglass is finished, flip the 11 min hourglass would have lost only 2 minutes till now. On inverting, we get to measure those two minutes.
Now 13 + 2 = 15 minutes.

In this way, fifteen minutes can be measured in just 3 steps.

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